How is the "right to explanation" reasonable?
How is the right to explanation reasonable, given the current standards at which we hold each other accountable?


In short, it is quite reasonable.

More specifically, making AI accountable and responsible for explaining the decision seems reasonable because


Humans (DARPA in this case) has chosen to create, raise and evolve AI with the tax-payers money. In our society as well, whenever humans have come together for some purpose (some formal arrangement like government or otherwise), accountability and responsibility is assumed and assigned. Thus, expecting AI (who will take decisions on our behalf) to explain their decisions seems only natural extension of how humans currently operate.
Humans (generally at least) don't have super-power or resources to control and manipulate the rest of the population. But when they do (like our political leaders), we want them to be accountable. 
In rare cases, powerful humans become powerful due to things which are accessible to everyone, while AI's super-power won't be. So, our civilization would feel safer and thus less insecure with an AI which doesn't shut the door on face when questions are asked.


Other benefits of AI that offers an explanation on how a decision is reached


Far easier to debug and improve in its early stage.
Can be customized further (and along the way) to amplify certain aspects like social-welfare over profitability, etc.